Prosecutors say Gray suffered a critical neck injury in April after being handcuffed and shackled in the back of a Baltimore police transport van that made several stops. Six police officers have been indicted on charges connected to his death.

The city's police department was quick to criticize the sign on the van door after the photos came to light Thursday.

"The nature and the posting of the wording is both concerning and unacceptable," Baltimore City Police Sgt. Jarron Jackson said.

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While the department said it was unable to confirm the authenticity of the photos, Jackson said officials are taking the issue "very seriously."

"Deputy Police Commissioner Dean Palmere has ordered that an inspection of all departmental vehicles be conducted," Jackson said. "Internal investigations will be initiated when appropriate."

CNN affiliate WBAL reported that the photos were taken Tuesday near the Central District Police Station.

Baltimore City Paper reported that it had spoken to the photographer, who asked not to be identified by name. She sent the newspaper an original version of a photo of the sign from her phone, it said.

WBAL said it was unclear who had posted the sign or how long it had been in place.